Adsorption of Cu2+ and Mg2+ ions on silica gel derived from rice hulls ash: Experimental and theoretical studies

Abstract

Experimental study and quantum chemical calculations have been used to study the adsorption of Cu2+ and Mg2+ on silica gel derived from rice hulls ash. XRD measurements showed that the silica obtained has amorphous structure with BET of 250.1[Formula: see text]m2/g and pzc at pH 2. Adsorption of Cu2+ and Mg2+ on silica at 28[Formula: see text]∘C showed maximum capacities of 0.26 and 0.20[Formula: see text]mmol/g, respectively. In both cases of metal ions, the interaction was explained to proceed via hydrogen bonding through the outer hydration shell. Using density functional theory (DFT), interactions of hydrated metal ions with naked, mono- and di-hydrated silanol have been investigated. Calculations at the used level predict an increase of the entropy upon adsorption interaction with dehydrated silanol. Adsorption of hydrated Cu2+ and hydrated/un-hydrated Mg2+ on silanol is an exothermic process as predicted by B3LYP/6-31+G(d)//HF/6-31+G(d). This, in turn, leads to more negative free energy of reaction compared to experiment, [Formula: see text]59.8 versus [Formula: see text]25.8[Formula: see text]kJ/mol for Cu2+ and [Formula: see text]53.8 versus [Formula: see text]22.6[Formula: see text]kJ/mol for Mg2+. However, the strength of interaction between silica and both ions was reproduced correctly by theory.